The use of fossil fuel as the combustible fuel in gas turbine engines results in the combustion products of carbon monoxide, carbon dioxide, water vapor, smoke and particulates, unburned hydrocarbons, nitrogen oxides and sulfur oxides. Of these above products, carbon dioxide and water vapor are considered normal and unobjectionable. In most applications, governmental imposed regulation have and are further restricting the amount of pollutants being emitted in the exhaust gases.
In the past the majority of the products of combustion have been controlled by design modifications. For example, smoke is normally controlled by design modifications in the combustor, particulates are normally controlled by traps and filters, and sulfur oxides are normally controlled by the selection of fuels being low in total sulfur. This leaves carbon monoxide, unburned hydrocarbons and nitrogen oxides as the emissions of primary concern in the exhaust gases being emitted from the gas turbine engine.
Oxides of nitrogen are produced in two ways in conventional combustion systems. For example, by the direct combination of atmospheric nitrogen and oxygen at the high temperatures occurring in the combustion zone and the presence of organic nitrogen in the fuel causes the production of nitrogen oxides. The rates with which nitrogen oxides form depend upon the flame temperature and, consequently, a small reduction in flame temperature will result in a large reduction in the nitrogen oxides.
Past and some present systems provide means for reducing the maximum temperature in the combustion zone of a gas turbine combustor have included schemes for introducing more air at the primary combustion zone, recirculating cooled exhaust products into the combustion zone and injecting water spray into the combustion zone. An example of such a system is disclosed in U.S. Pat. No. 4,733,527 issued on Mar. 29, 1988 to Harry A. Kidd. The method and apparatus disclosed therein automatically maintains the NOx emissions at a substantially constant level during all ambient conditions and for no load to full load fuel flows. The water/fuel ratio is calculated for a substantially constant level of NOx emissions at the given operating conditions and, knowing the actual fuel flow to the gas turbine, a signal is generated representing the water metering valve position necessary to inject the proper water flow into the combustor to achieve the desired water/fuel ratio.
Another example of a method and apparatus for reducing NOx emissions is disclosed in U.S. Pat. No. 4,215,535 issued on Aug. 5, 1980 to George D. Lewis. In this patent, the apparatus has a combination of serpentine geometried, fuel-mixing tubes discharging to the radially outward area of the combustor and an axially oriented, fuel-mixing tube near the center of the combustor are adapted to generate a strong centrifugal force field within the combustor. The tube near the center has a convergent section and a divergent section. A fuel supply means discharges fuel into the convergent section wherein vaporization of liquid fuel is aided by a differential axial velocity over the length of the tube. The force field promotes rapid mixing and combustion within the chamber to reduce both the magnitude of the combustor temperature and the period of exposure of the medium gases to that temperature, thus, reducing the formation of NOx.
Another method for reducing the formation and emission of NOx is disclosed in U.S. Pat. No. 3,842,597 issued Oct. 22, 1974 to Frederic Franklin Ehrich. This patent teaches a means for bleeding and cooling a portion of the airflow pressurized by the compressor which is then introduced into the primary combustion zone of the combustor in order to reduce the flame temperature effecting a reduction in the rate of formation of oxides of nitrogen.
The above systems are examples of attempts to reduce the emissions of oxides of nitrogen. Many of the attempts have resulted in additional expensive components. For example, the Kidd concept requires an additional means for injecting water into the combustion chamber which includes a water source, a control valve, a controlling and monitoring system and a device for injecting water into the combustion chamber. The Lewis concept requires a plurality of fuel-mixing tubes or injectors, a control system for each tube and a monitoring system with feedback to each of the controls of individual tubes. The Ehrich concept requires additional components to bleed and cool a portion of the airflow pressured by the compressor and hardware to reintroducing the cooled air into the combustor.